Effect of Rainfall and Urban Expansion on Runoff in the Agricultural Area Thanutch Sukwimolseree Department of Civil Engineering, Kasetsart University, Bangkok, Thailand Email: fengtnsr@ku.ac.th Preeyaphorn Kosa School of Civil Engineering, Suranaree University of Technology, Nakhon Ratchasima, Thailand Email: kosa@sut.ac.th Abstract—In Thailand, agricultural area is very important for a main occupation of Thai people. However, the agricultural areas changed to be the urban areas. Then, the objective of this study is to study the effect of variation rainfall and urban expansion on runoff. A study area is an agricultural area in the Mun river basin. To achieve purpose, there are two parts of this study. They consists of (1) the calculation of daily runoff based on land use 2008 and (2) the daily runoff determination based on the variation of rainfall and urban development . The results present that the runoff increases from the case of urban explanation 5 km to 15 km. A big change of runoff reveals from the case of land use 2008 to the case of urban explanation 5 km. Moreover, mean monthly runoff base on rainfall more than 100 mm/month is higher than mean monthly runoff base on rainfall less than 100 mm/month. The high runoff occurs during May to October. There is the lowest mean runoff in the urban area 1.00-25.00% but there is the highest mean runoff in the urban area 50.01-75.00%. The difference of runoff in the urban area 1.00-25.00% and in the urban area 50.01-75.00% is range 0.35 – 47.12 mm. or 13.97 mm. by averaging. The average runoff has a direct variation with the rainfall and urban area. It means that increased urban area can indicate a variation of runoff . In addition, rainfall is directly affect to runoff. Index Terms—rainfall, runoff, urban area, agricultural area, SWAT 1 I. INTRODUCTION Naturally, the hydrological cycle is a mainly process in an agricultural area. The components of hydrological cycle include rainfall, runoff, groundwater, evapotranspiration, infiltration, percolation, interception, water vapor, overland flow, subsurface flow, and influent stream. Then, water is a significant production factor in the agriculture. The both of climate change and land use change have an important effect on agricultural area in the term of runoff [1]-[3]. For climate change, the runoff can be calculated by the assessment of various global, region and nation using existing global model and Manuscript received June 8, 2019; revised November 13, 2019. analyzing the historical data [4]-[8]. For land use change, the effect on runoff depends on the area size of land cover and average slope [9]. The peak runoff can be raised by increased city area while it can be decreased by decreased green area [10]. In Thailand, agricultural area is very important because it is a main land use type and a main occupation of Thai people. Thailand has an area of about 513,000 square kilometers. The area for agriculture is about 43% (138 million hectares) and agriculture accounts for 10% of GDP. Normally, water for agricultural area is from irrigation and rainfall but it is mainly from rainfall . Since rainfall is the fall and the main driver of the water cycle over land, runoff is the excess water that flows to an agricultural area and into rivers. However, economic expansion and residential business have led to agricultural areas being compromised. The agricultural areas changed to be the urban areas. Then, the objective of this study is to study the effect of variation rainfall and urban expansion on runoff. A study area is an agricultural area in the Mun river basin. A. Mun River Basin The Mun River Basin locates in the lower part of north east of Thailand and a river basin area is about 69,700 square kilometers. This river basin locates from 14 o 07’ N to 16 o 21’ N and from 101 o 17’ E to 105 o 38’ E. It covers 24 sub-river basins shown in Fig. 1. Land use in the Mun river basin presents in Table I. The main river is Mun River and it has a total length of about 726 kilometers. The amount of water flowing into the Mekong River is approximately 26,655 million cubic meters a year. There are four big reservoirs included Lamtakong, Lam Phra Phloeng, Upper Mun, and Lam Nangrong reservoirs. TABLE I. LAND USE IN MUN RIVER BASIN Land use Type Area in sq.km Area in % Agricultural area (A) 14453.53 69.73 Forest (F) 3718.48 17.94 Urban area (U) 1264.99 6.10 Water area (W) 453.62 2.19 Others area (M) 836.94 4.04 281 Journal of Advanced Agricultural Technologies Vol. 6, No. 4, December 2019 ©2019 Journal of Advanced Agricultural Technologies doi: 10.18178/joaat.6.4.281-286